The first definition indicates a crocodile of the Indian sub-continent. India indeed has a mugger crocodile (crocodylus palustris).

The second definition is a noun - ‘one who attacks with intent to rob’. The first known use listed is 1863.

What is the history of the second definition? Could it be a vernacular that was used by the British military during its occupation in India?

I once heard a radio discussion (perhaps originally a Reader’s Digest story) that implied the term ‘mugger, or mugged’ was British military slang. As the story went, sometimes British soldiers on R & R would be jumped by thieves. After a good thumping, the body of the dead or unconscious soldier would be dumped into nearby rivers or swamps where he became fodder for the mugger crocodiles. When the soldier could not be found during roll-call the following morning, a search would ensue. If the missing person wasn’t found, he was marked down as having been ‘mugged’.

Any validity to this explanation, or is this just another popular myth? Thank you, Ulli

The verb to mug, meaning to rob, comes from boxing. It originally meant to strike an opponent in the face, or mug. This sense dates to around 1818. Within a few decades it had come to mean to strike someone with the intent to rob them. If you look at Merriam-Webster closely, you’ll note they give a different origin for each of the two senses.